(1) Field of the Invention
The present invention relates to piezoelectric porcelains suitably for a member of a piezoelectric actuator, a piezoelectric buzzer and so on which utilize piezoelectric distortion.
(2) Related Art Statement
Lead zirconate titanate has a large piezoelectric distortion and can be used at a high temperature. Moreover, if a part of zirconate or titanate is substituted by the other elements, various kinds of properties can be obtained. Particularly, piezoelectric porcelains of three component series having composition of PbTiO.sub.3 -PbZrO.sub.3 -Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3, in which a part of zirconate or titanate is substituted by magnesium and niobium, has a relatively large piezoelectric constant d.sub.31.
Recently, in fields of precision machines, an optical instruments and so on, precise position control is needed. To this end, a piezoelectric element utilizing a piezoelectric distortion has been developed. As for such elements, a piezoelectric porcelain having a high piezoelectric constant d.sub.31 is desired so as to make a distortion amount of the element larger. However, from the point of view mentioned above, a piezoelectric constant d.sub.31 of the piezoelectric porcelain of three component series having a composition of PbTiO.sub.3 -PbZrO.sub.3 -Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 mentioned above is -225 pm/V and thus it is not sufficient for this purpose.
Further, in Japanese Patent Laid-Open Publication No. 3-40964, it is disclosed that an excellent property such as a piezoelectric constant d.sub.31 =-307.9 pm/V can be realized in a piezoelectric porcelain of three component series having a composition of Pb(Mg.sub.1/3 Nb.sub.2/3).sub.0.225 (Ni.sub.1/3 Ta.sub.2/3).sub.0.15 Ti.sub.0.375 Zr.sub.0.25 O.sub.3.
However, in the piezoelectric porcelain having the composition disclosed in Japanese Patent Laid-Open Publication No. 3-40964, a Curie point is 177.degree. C. which is very low. If the Curie point of the piezoelectric porcelain is low in a piezoelectric element, a limit of temperature, to which the piezoelectric porcelain is driven, becomes low. That is to say, if a Curie point of the piezoelectric porcelain is low, the following disadvantages occur.
Generally, since a piezoelectric porcelain after firing shows no piezoelectricity, it is necessary to perform a poling thereto under applying a DC voltage. However, if the piezoelectric porcelain after the poling mentioned above is once used at a temperature above a Curie point, the piezoelectric porcelain shows no piezoelectricity again. Therefore, it is necessary to perform a poling again for the piezoelectric porcelain. In this manner, the piezoelectric porcelain having a composition of Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 -Pb(Ni.sub.1/3 Ta.sub.2/3)O.sub.3 -PbTiO.sub.3 -PbZrO.sub.3 has a high piezoelectric property, but the use of the piezoelectric porcelain mentioned above is limited to low temperatures.